Noob question about the bitscope schematic

The JFET is a high impedance input stage. This is required on a CRO in order to get the standard 1Mohm input impedance provided by R26. The opamp does not have high enough input impedance to do this, if it did, then yes, you could connect direct to the opamp.

This is a common circuit for CRO front ends. Q3 is usually a resistor in the basic implementation. Q3, RV3 and C35 could be removed, but the FETs would have to be be extremely well matched electrically and thermally to ensure no offset.

Dave :)

I forget to add that, yes Q3 could be replaced by a diode. But either way I can't see why Q3 is needed, it will in fact add some temperature dependency to the DC offset, that's bad. Replacing Q3 with a resistor (the traditional circuit topology) ensures no temperature related DC offset (assuming RV3 has the same tempco). But even if they are different tempcos it's probably going to be negligable. Using Q3 on the other hand will I suspect add quite substantial DC offset with temperature change.

Dave :)

"David L. Jones"

The trim pot merely sets the current flowing through Q7 and hence also Q6.

When that current level is such that the *bias* voltage between the gate and source of Q6 is *equal* to the Vbe of Q3, then the output rests at zero volts.

........ Phil

"David L. Jones"

....... Phil

I was referring to the more basic (standard) circuit that has no resistors (or Q3 for that matter), just the two FETs. In this case the two FETs must be matched in order not to get any DC offset on the output. Adding the resistors (or VR3 + Q3) allows the use of non-matched FETs, but trimming is required for the DC offset.

Adding the transistor Q3 appears to achieve nothing but give you an (unwanted) offset variation with temperature. Changing Q3 to a resistor avoids any variation with temperature.

Dave :)

"David L. Jones"

..... Phil

Shame I did. What part of "Q3, RV3 and C35 could be removed" didn't you understand? If you remove those components you have nothing left but the two FETs.

Perhaps that was the intention, but I recon it might cause more DC offset variation with temp of its own accord. The two FETs will after all be reasonably matched in that respect.

Dave :)

"David L. Jones"

Try learning how to write someday, Dave - it is *never* too late.

FACT:

YOU wrote this:

" Q3 is usually a resistor in the basic implementation."

That makes on resistor, at least.

NOW you try to bullshit everyone by saying there are NO resistors in your so called "basic implementation" ???????????

BTFW

If Q3 is a fixed resistor, then a second one of similar value one HAS to be placed in the drain cct of Q7.

That makes TWO resistors as well as matched FETs for zero offset.

Piiisssss off - wanker.

....... Phil

No verbal ploy. I was clearly talking about a two FET circuit without any resistors. Not my fault if you can't comprehend that.

Sounds like you are the one who is getting tedious again Phil.

I wrote this: "Q3, RV3 and C35 could be removed, but the FETs would have to be be extremely well matched electrically and thermally to ensure no offset."

That was a seperate sentence, and clearly meant a circuit with only the FETs.

You are either confusing the two circuits or just deliberately playing around to prove I'm wrong in some way - I suspect the later. I said the circuit with the resistor replacing Q3 is the "basic implementation", because that is usually the basic practical implemention. This circuit has two resistors.

The circuit with the two FETs and no resistors I called a "more basic (standard) circuit ".

Yes it does, I did not say otherwise. I said that Q3 can be replaced by a resistor, VR3 is still in there. Doing this requires the value to be trimmed as in the Bitscope design.

As I was trying to point out, you can also get zero offset with *no resistors*, BUT the two FETs must be an electrically and thermally matched pair. That is the most basic circuit, but it is not often implemented in practice. The one with the two resistors is the standard way to implement this in practice.

Resorted to bad language again I see *sigh*

Of course you can't prove I'm wrong technically so that's what you have to resort too. Sad.

Dave :)

Hi Dave, You'll have to excuse Phil, It's a full moon again and everyone at aus.hi-fi has left to move to a moderated group where He's not allowed. As he's got nowhere to go he's turned up here to cause trouble.

What? Her menstrual cycle is in phase (-lock) with the moon? Awesome.

Heh heh, the term "Lunatic" has a basis in fact.